1. Field of the Invention
The present invention relates to a method of fabrication of an x-ray image intensifier as well as to the x-ray image intensifiers thus obtained.
2. Description of the Prior Art
X-ray image intensifiers, also designated as X.I.I. tubes, are well-known in the prior art. Their function is to convert an x-ray image to a visible image for such purposes as medical observation, for example.
It is recalled that an X.I.I. tube as shown diagrammatically in longitudinal cross-section in FIG. 1 consists of an input screen, an electron-optical system and a viewing screen which are contained within a vacuum enclosure 1.
The input screen is provided with a scintillator 2 for converting the incident x-ray photons to visible photons and a photocathode 3 for converting the visible photons to electrons. There is usually interposed between the scintillator and the photocathode an electrically conductive sub-layer having the function of re-supplying the photocathode with electric charges during emission of its electrons. This sub-layer is not shown in FIG. 1.
The scintillator can consist, for example, of sodium or thallium doped cesium iodide. The photocathode can be formed of an alkali antimonide corresponding, for example, to the formula Sb Cs.sub.3, Sb K.sub.3, Sb K.sub.2 Cs . . . . By way of example, the conductive sub-layer can be formed of indium oxide having the formula In.sub.2 O.sub.3.
The electron-optical system is usually constituted by three electrodes G.sub.1, G.sub.2, G.sub.3 and by an anode A which carries the viewing screen 4.
The photocathode 3 is usually connected to the ground of the tube. The electrodes G.sub.1, G.sub.2, G.sub.3 and the anode A are brought to electric potentials which rise to a value of 30 KV, for example. An electric field E is accordingly produced within the tube and is directed along the longitudinal axis of the tube towards the photocathode. The electrons emitted from the photocathode pass upstream through said field and strike the viewing screen 4 which is formed of cathodoluminescent material such as zinc sulphide, for example, thus making it possible to obtain a visible image.
The problem which arises and for which the present invention offers a solution is that objectionable parasitic illumination of the viewing screen is observed in X.I.I. tubes, even in the absence of x-radiation. This parasitic illumination is due to the alkali metals which are unintentionally deposited on the electrodes of the X.I.I. tube at the time of fabrication of the photocathode. The intense electric field which prevails within the tube has the effect of stripping electrons from these alkali metals which are highly electropositive and therefore very readily ionizable. These electrons move upstream through the electric field, strike the viewing screen and produce parasitic illumination.
This phenomenon is illustrated in FIG. 2 which is a part-sectional view of the grid G.sub.3 and of the anode A of the X.I.I. tube of FIG. 1. The reference numeral 7 designates the alkali metal layer which has been deposited on the grid G.sub.3. Under the action of the electric field E which is maintained between the grid G.sub.3 and the anode A and which is directed towards the grid G.sub.3, said layer liberates electrons which move upstream through the electric field and strike the viewing screen 4.
It must be realized that the fabrication of photocathodes of the alkali antimonide type is performed within the vacuum enclosure of the X.I.I. tube since alkali metals are highly reactive and have to be formed in vacuo in order to be stable. These photocathodes can be fabricated by successive evaporations of their constituent elements. To this end, there is placed within the tube an antimony generator consisting of an ordinary crucible which contains antimony and in which evaporation is produced by heating the crucible by Joule effect, for example. The antimony generator 5 is usually placed in proximity to the photocathode and on the path of the electrons as shown in FIG. 1. It is for this reason that the generator is usually removed from the enclosure once the photocathode has been completely formed. The alkali metals are evaporated from alkali-metal generators 6, these generators being usually located on the electrode G.sub.3 which is nearest the anode A as shown in FIG. 1.
The alkali-metal generators are usually left within the vacuum enclosure once the photocathode has been completed. In some known methods of fabrication of X.I.I. tubes, the alkali-metal generators are not carried by the electrode G.sub.3 and are removed from the vacuum enclosure when fabrication of the photocathode has been completed.
Evaporation of the alkali metals is the result of a silicothermic reaction or aluminothermic reaction in the presence of chromates of the metals to be evaporated. The silicothermic or aluminothermic reactions are initiated by Joule heating of the alkalimetal generators.
The alkali-metal generators just mentioned are much less directional than the antimony generators. This is due to the fact that, in order to ensure that the silicothermic or aluminothermic reactions take place under good conditions, it is necessary to employ special crucibles in which the chromates are confined. This type of crucible exhibits poor directivity which has the advantage of ensuring wholly uniform deposition of the alkali metals over the entire surface of the photocathode which is located at a distance from these crucibles 6. The disadvantage of this crucible, however, lies in the fact that it causes deposition of alkali metals on all the parts of the X.I.I. tube and in particular on the electrodes G.sub.1, G.sub.2 and G.sub.3, thus giving rise to the problem of parasitic illumination of the viewing screen.
In order to solve this problem, one solution adopted by the present Applicant is to form an oxide coating on the electrode G.sub.3 which is usually formed of aluminum.
This solution does in fact eliminate the problem of parasitic illumination of the viewing screen but introduces discharges through the oxide coating or layer.
When the X.I.I. tube receives x-radiation, a part of the electrons emitted from the photocathode falls on the electrode G.sub.3. Since the electrode G.sub.3 is coated with an oxide layer, these electrons do not flow, thus giving rise to discharges through the oxide layer.